The blog of dlaa.me

A few months ago, I dashed out a JavaScript function to efficiently generate a random (version 4) UUID per RFC 4122. This was inspired by code I saw for the same purpose that was not especially efficient - or random. (For more on randomness in JavaScript, see my post about polyfilling Math.random().) My UUID generator uses crypto.getRandomValues() for the best randomness and tries to do as little extra work as possible for efficiency.

The generateRandomUUID page on GitHub has the code and a link to the generateRandomUUID sample page that runs in your browser and generates a new UUID every time it's loaded.

Because it's so small, I've included the code here for reference:

// generateRandomUUID.js
// https://github.com/DavidAnson/generateRandomUUID
// 2019-10-27

"use strict";

// JavaScript code to efficiently generate a random UUID per RFC 4122
function generateRandomUUID() {
  // UUIDs have 16 byte values
  const bytes = new Uint8Array(16);
  // Seed bytes with cryptographically random values
  crypto.getRandomValues(bytes);
  // Set required fields for an RFC 4122 random UUID
  bytes[6] = (bytes[6] & 0x0f) | 0x40;
  bytes[8] = (bytes[8] & 0x3f) | 0x80;
  // Convert bytes to hex and format appropriately
  const uuid = Array.prototype.map.call(bytes, (b, i) => {
    // Left-pad single-character values with 0,
    // Convert to hexadecimal,
    // Add dashes
    return ((b < 16) ? "0" : "") +
      b.toString(16) +
      (((i % 2) && (i < 10) && (i > 2)) ? "-" : "");
  }).join("");
  // Return the string
  return uuid;
}